METHOD FOR NONDESTRUCTIVE LIFT-OFF OF GaN FROM SAPPHIRE SUBSTRATE UTILIZING A SOLID-STATE LASER

ABSTRACT

A method for nondestructive laser lift-off of GaN from sapphire substrates utilizing a solid-state laser is disclosed in the present invention, wherein, a solid-state laser is used as the laser source, and a small laser-spot with a circumference of 3 to 1000 micrometers and a distance of two farthest corners or a longest diameter of no more than 400 micrometers is used for laser scanning point-by-point and line-by-line, wherein the energy in the small laser-spot is distributed such that the energy in the center of the laser-spot is the strongest and is gradually reduced toward the periphery. According to the present invention, a nondestructive laser lift-off with a small laser-spot is achieved, and a scanning mode of the laser lift-off is improved, thereby a lift-off method without the need of aiming is achieved. As a result, the laser lift-off process is simplified, and the efficiency is improved while the rejection rate is reduced, such that the obstacles of the industrialization of the laser lift-off process are removed.

TECHNICAL FIELD

The present invention relates to methods for preparations of GaNsubstrates and related devices by laser lift-off, more specifically toapplications of solid-state lasers to separate GaN and sapphiresubstrate, by which a process for scanning is improved, and a methodwithout the need of aiming is achieved, thereby preparations for GaNsubstrates and related devices are achieved.

BACKGROUND OF THE INVENTION

In recent years, III/V nitrides, dominated by GaN, InGaN, and AlGaN, arereceiving high attentions as semiconductor materials. Thanks to theircharacteristics of continuously variable direct band gap from 1.9 to 6.2eV, excellent physical and chemical stability, and high saturationelectron mobility etc., they are the most preferred materials foroptoelectronic devices such as laser devices and light-emitting diodes.

However, as limited by the growth technologies of GaN itself, large areaGaN materials nowadays are mostly grown on sapphire substrates. AlthoughGaN grown on sapphire substrates has high quality and wide applications,the development of GaN based semiconductor devices is largely limited bythe non electro-conductivity and poor thermal-conductivity of sapphires.In order to avoid such disadvantages, methods to replace sapphires,after growth of GaN based devices on sapphires, with substrates of Si,Cu or the like showing high thermal-conductivity and highelectro-conductivity were invented. During the removal of sapphires, amainly applied method is the laser lift-off technology.

A laser lift-off technology is a method to irradiate the GaN layerthrough sapphire substrates at the joint of sapphire-gallium nitride,with a laser source whose energy is less than the band gap of sapphirebut larger than the band gap of GaN, as a result of the GaN hereabsorbing the laser energy and yielding high temperature, it isdecomposed into gallium and nitrogen gas, so that a separation of GaNand sapphire substrate is achieved.

Conventional laser lift-off technologies use approaches with largelaser-spot (circumference of the laser-spot is larger than 1000micrometers), scanning chip by chip for lift-off, to achieve separationof GaN based devices and sapphire substrates. The disadvantages of suchlarge laser-spot lift-off technologies are that, because of the largefluctuation of energy on the edge of the laser-spot, stress is highlyconcentrated on the edge, resulting in that GaN at the edge of thelaser-spot is seriously damaged, as shown in FIG. 1. The damage depthcan be unequally from zero point several micrometer to severalmicrometers, and unavoidable. In this way, the process for laserlift-off of GaN based devices is seriously confined.

Presently, a process for making laser lift-off GaN based devices underthis condition approximately are:

(1) A GaN based epitaxial wafer is grown on a sapphire;

(2) The epitaxial wafer with the sapphire substrate is made into GaNbased separated device cells;

(3) Other thermal-conductive and electro-conductive substrates areelectroplated or bonded;

(4) The sapphire substrate is removed by a laser lift-off method.

In the above-mentioned process, in order to avoid the large laser-spotedge damage to the GaN based devices (generally there are two types:millmeter magnitude high power devices and micrometer magnitude lowpower devices, with various sizes), the mostly adopted method is todirectly cover a whole of one or more GaN based device cells, and todispose the laser-spot edge in passages between GaN based device cells,so as to avoid laser-spot edge damages as much as possible.

The disadvantages of doing so are: (1) the laser-spot area has to beadjusted in accordance with the device size; (2) repeated aimings haveto be done before laser scanning, to ensure that the laser-spot edge isin passages between GaN device cells; (3) a real-time video trackdetection is needed, in order that once a deviation arises in thescanning, it is stopped immediately for correction. The abovedisadvantages bring about great obstacles to large scale productionapplications of the laser lift-off process, and largely complicated theprocess, resulting in reduced efficiency and increased rejection rate(because of deviations in laser scanning processes, the edge damages areaggravated).

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a laser lift-offmethod without the need of aiming, achieving a nondestructive lift-offof GaN from sapphire substrates.

The technical solution of the present invention is as follows:

A method for laser lift-off of GaN from sapphire substrates,characterized in that, a solid-state laser is used as the laser source,and a small laser-spot with a circumference of 3 to 1000 micrometers anda distance of two farthest corners or a longest diameter of no more than400 micrometer is used for laser scanning point-by-point andline-by-line, wherein the energy in the small laser-spot is distributedsuch that the energy in the center of the laser-spot is the strongestand is gradually reduced toward the periphery.

According to the present invention, large laser-spot lift-off technologyin prior laser lift-off methods is changed by using small laser-spot toachieve a method to lift-off GaN based device in needless of aiming. Twoimportant reasons that the small laser-spot method has not been advancedare: (1) it is generally recognized that an edge problem will beintroduced into cells of the GaN based devices by small laser-spotlift-off, so that the quality of laser lift-off will be reduced; (2)nondestructive laser lift-off by small laser-spot has not been realizedyet.

A circumference of the small laser-spot used in the present invention is3 to 1000 micrometers, and a distance of two farthest corners or alongest diameter is not more than 400 micrometers. Preferably, thecircumference is 100 to 400 micrometers, and a distance of two farthestcorners or a longest diameter is no more than 150 micrometers. The shapeof the small laser-spot can be square, rectangle, circle, oval,pentagon, hexagon, heptagon or octagon, and so on. Such small laser-spotare for example a square laser-spot with a length of each side of 1 to250 micrometers, or a circle laser-spot with a diameter of 1 to 400micrometers.

At the same time, the present invention made adjustments to the laserenergy distribution of a single laser-spot, so that the prior internallaser energy fluctuation condition of the laser-spot is changed. Inprior arts, energy in the large laser-spot is uniform, but with abruptchanges in the edge of the laser-spot, therefore damage is easilyproduced. Prior energy distribution of pulse laser-spot is shown in FIG.2 a, in which the X-axis represents the side length direction of thelaser-spot, and the Y-axis the energy scale, and the zero position ofthe X-axis corresponds to the center of the laser-spot. In the presentinvention, internal energy distribution condition of the laser-spot ischanged and an energy uniformization is no longer pursued, while agradual change of energy at the edge of the laser-spot is taken intoconsideration. The energy distribution can be as shown in FIG. 2 b.

Relative to the large laser-spot, the small laser-spot is easier toachieve a gradual change of laser energy of the laser-spot. It is theemphasis on the energy gradual transition (gradually changed from astronger energy section close to the center of the laser-spot to aweaker energy section far from the center) in the edge section of thelaser-spot that the stress condition of the GaN based material at theedge of the laser-spot is improved, thereby a nondestructive laserlift-off by small laser-spot is achieved.

The solid-state laser used in the present invention can be an improvedsolid frequency multiplier laser source, which is improved in laserfluctuation condition inside the laser-spot, that the energy is thehighest in the center of the laser-spot and is gradually reduced towardthe periphery, and the internal energy throughout the laser-spotrepresents a Gauss distribution or approximately a Gauss distribution.

According to the present invention, a nondestructive laser lift-off bysmall laser-spot is achieved (lift-off surface as shown in FIG. 3,without apparent damage), thereby a lift-off method without aiming isachieved. According to the method of the present invention, the laserlift-off scanning mode is improved. After a step of electroplating orbonding by traditional processes, it is no longer needed to adjust thelaser-spot area in accordance with the GaN device cell size, and a chipby chip scanning is not necessary. The laser scanning can be directlycarried out without intermediate pauses or real time detections.

In comparison with prior arts, the advantageous effects of the presentinvention are:

Firstly, the laser lift-off process is largely simplified;

Secondly, the operating efficiency of laser lift-off is largelyimproved;

Thirdly, the rejection rate is reduced;

Fourthly, obstacles to the industrialization of the laser lift-offprocess are removed, and industrial production of the laser lift-off isadvanced.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photo of the damage to laser edges left on GaN after largelaser-spot laser lift-off.

FIG. 2 a is a schematic diagram of the internal energy distribution of alaser-spot in prior arts; FIG. 2 b is a schematic diagram of theinternal energy distribution of a small laser-spot in the technicalsolution of the present invention.

FIG. 3 is a photo of a specimen surface after small laser-spot lift-offof the present invention.

FIG. 4 is a microscope photo magnified by 500 times of a device surfaceafter laser lift-off of example 1 of the present invention.

FIG. 5 is a microscope photo magnified by 500 times of a device surfaceafter laser lift-off of example 2 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is further described in details by examples inconjunction with figures, but not in any way limiting the presentinvention.

Example 1

A vertical structure of a GaN based device is made by laser lift-offaccording to the following steps:

(1) On a sapphire, a GaN based epitaxial wafer is grown, and furthermade into GaN based separated device cells. Then a Cu layer is bonded onthe GaN surface by Pd/In bonding method, wherein Pd is plated bysputtering to 200 nm, and In is evaporated by thermal evaporation to 600nm. Then the Cu layer plated with Pd/In metal and the GaN wafer arebonded at a high temperature of 200° C., and a high pressure of 1 mPa,for a duration of 20 minutes;

(2) A lift-off of the bonded specimen using an improved solid frequencymultiplier laser source (YAG laser source), with a laser energy densityof 600 mj/cm², a laser frequency of 20 Hz, a laser-spot size of 30micrometers in square, and a laser-spot distance between centers of 30micrometers, is carried out, and a point-by-point and line-by-linescanning is carried out at the sapphire surface of the specimen usingthe laser.

After the lift-off, the sapphire substrate is removed. The lift-offresult is shown as FIG. 4, which shows in a magnification of 500 timesthat the lift-off surface of the device is uniformly nondestructive.

Example 2

A vertical structure of a GaN based device is made by laser lift-offaccording to the following steps:

(1) On a sapphire, a GaN based epitaxial wafer is grown, and furthermade into GaN based separated device cells. Then a Cu layer is bonded onthe GaN surface by Pd/In bonding method, wherein Pd is plated bysputtering to 200 nm, and In is evaporated by thermal evaporation to 600nm. Then the Cu layer plated with Pd/In metal and the GaN wafer arebonded at a high temperature of 200° C., and a high pressure of 1 mPa,for a duration of 20 minutes;

(2) A lift-off of the bonded specimen using an improved solid frequencymultiplier laser source (YAG laser source), with a laser energy densityof 600 mj/cm², a laser frequency of 20 Hz, a laser-spot size of 100micrometers in square, and a laser-spot distance between centers of 100micrometers, is carried out, and a point-by-point and line-by-linescanning is carried out at the sapphire surface of the specimen usingthe laser.

After the lift-off, the sapphire substrate is removed. The lift-offresult is shown as FIG. 5, which shows in a magnification of 500 timesthat the lift-off surface of the device is uniformly nondestructive.

1. A method for non-destructively lifting a III-V semiconductor devicefrom a sapphire substrate, comprising: obtaining a III-V semiconductorlayer grown on a sapphire substrate at an interface; irradiating a laserbeam through the sapphire substrate, wherein the laser beam forms alaser spot at the interface between the III-V semiconductor layer andthe sapphire substrate, wherein the laser spot has a distance betweentwo farthest corners or a longest diameter smaller than 400 μm; andscanning the laser beam across the interface between the III-Vsemiconductor layer and the sapphire substrate to form a plurality oflaser spots at the interface to non-destructively separate the III-Vsemiconductor layer from the sapphire substrate.
 2. The method of claim1, wherein the perimeter of the laser spot has a distance between twofarthest corners or a longest diameter no more than 150 micrometers. 3.The method of claim 1, wherein the perimeter of the laser spot has alength between about 3 μm and 1000 μm.
 4. The method of claim 13,wherein the perimeter of the laser spot has a length between about 100μm and 400 μm.
 5. The method of claim 1, wherein the laser spot at theinterface has a width of about 30 microns μm. 6-20. (canceled)
 21. Themethod of claim 1, wherein the laser spot at the interface has a widthof about 100 microns μm.
 22. The method of claim 1, wherein the distancebetween the centers of adjacent laser spots is about the same as thewidth of the laser spot.
 23. The method of claim 1, wherein the distancebetween the centers of adjacent laser spots at the interface is in arange of about 30 μm and about 100 μm.
 24. The method of claim 1,wherein the laser spot at the interface has a shape selected from thegroup consisting of a square, a rectangle, a polygon, a circle, and anelliptical.
 25. The method of claim 1, wherein the laser spot has asubstantially Gaussian energy distribution at the interface between theIII-V semiconductor layer and the sapphire substrate.
 26. The method ofclaim 1, wherein the laser beam is emitted by a solid-state laserdevice.
 27. The method of claim 1, further comprising: bonding aconductive layer on a surface of the III-V semiconductor layer opposingthe interface between the III-V semiconductor layer and the sapphiresubstrate.
 28. The method of claim 27, wherein the conductive layercomprises copper, the method further comprising: depositing a palladiumlayer on the surface III-V semiconductor layer; depositing an Indiumlayer on the palladium layer; and bonding the conductive layer to theindium layer.
 29. The method of claim 28, wherein the palladium layer isdeposited on the surface III-V semiconductor layer by sputtering,wherein the Indium layer is deposited on the palladium layer by thermalevaporation.
 30. The method of claim 1, wherein the III-V semiconductormaterial comprises GaN.
 31. The method of claim 1, wherein the III-Vsemiconductor material comprises a nitride material.
 32. A method fornon-destructively lifting a III-V semiconductor device from a sapphiresubstrate, comprising: obtaining a III-V semiconductor layer grown on asapphire substrate at an interface; irradiating a laser beam through thesapphire substrate, wherein the laser beam forms a laser spot at theinterface between the III-V semiconductor layer and the sapphiresubstrate, wherein the perimeter of the laser spot has a length betweenabout 3 μm and 1000 μm; and scanning the laser beam across the interfacebetween the III-V semiconductor layer and the sapphire substrate to forma plurality of laser spots at the interface to non-destructivelyseparate the III-V semiconductor layer from the sapphire substrate. 33.The method of claim 32, wherein the perimeter of the laser spot has alength between about 100 μm and 400 μm.
 34. The method of claim 32,wherein the laser spot has a distance between two farthest corners or alongest diameter smaller than 400 μm.
 35. The method of claim 32,wherein the laser beam is emitted by a solid-state laser device.